1. Field of the Invention
This invention relates to the polymerization of hydroxyl-endblocked linear polydiorganosiloxane to higher molecular weights while in the presence of fillers through the use of basic diorganosilanolates as catalysts.
2. Description of the Prior Art
Polydiorganosiloxane polymers are prepared by polymerization of monomers, neutralization of the catalyst, then stabilization of the polymer. If a filled polymer is desired, a filler is added to the neutralized polymer. When a reinforcing filler is added to a neutralized polymer, the polymer and filler may react to make a tough, nervy mass that is difficult to further process. In order to prevent this reaction, known as crepe aging, many methods are known for treating the reinforcing filler either before use or in situ, so that the mixture has a suitable storage life.
The incorporation of estersils into elastomers is taught by Iler in U.S. Pat. No. 2,727,876, issued Dec. 20, 1955. He identifies estersils as an organophilic solid in a supercolloidal state of subdivision, having an internal structure of inorganic siliceous material with a specific surface are of at least 1 m.sup.2 /g, having chemically bound to said internal structure --OR groups wherein R is a hydrocarbon radical having from 2 to 18 carbon atoms. Iler notes that an estersil may be incorporated into a silicone during the formation of the elastic gum. He points out that it is customary to mill silicones with catalyst during the polymerization to form elastic gums, as shown in Agens U.S. Pat. No. 2,448,756, issued Sept. 7, 1948, and estersils may advantageously be milled into the silicone at this stage. The Agens patent teaches a process of making a solid, elastic, curable methylpolysiloxane by condensing a liquid polymeric dimethylsiloxane with an iron halide such as ferric chloride.
There are many known methods of preparing polydiorganosiloxane polymers. Hyde in U.S. Pat. No. 2,490,357, issued Dec. 6, 1949, teaches a method in which cyclic diorganosiloxanes are contacted with an alkali metal hydroxide. Kantor et al. in U.S. Pat. No. 2,883,366, issued Apr. 21, 1959, discloses a method of increasing the molecular weight of an organopolysiloxane by contacting an organopolysiloxane with a quaternary phosphonium compound until an increase in molecular weight is effected, then heating the resulting product to decompose the quaternary phosphonium compound. Brown et al. in U.S. Pat. No. 4,008,261, issued Feb. 15, 1977, describe a method of making phosphorus-containing catalysts suitable for polymerizing polyorganosiloxanes. Ostrozynski in U.S. Pat. No. 3,477,988, issued Nov. 11, 1969, teaches that quaternary bases, such as tetraalkylphosphonium silanolate are useful catalysts for rearrangement of organopolysiloxanes. He teaches that the rearrangement rates are increased by use of another organophosphorus promoter compound. He teaches that cyclic and other low molecular weight polysiloxanes can be rearranged with or without endblocking units to higher molecular weight polymers. High molecular weight polysiloxane can be arranged to lower polymers. His rearrangement method using base catalysts and his promoter is taught to inhibit silanol condensation in combination with diorganocyclosiloxanes and water. One of his embodiments teaches rearrangement in the presence of filler, for example low molecular weight diorganocyclosiloxane and silica filler are polymerized by a base-catalyzed rearrangement in the presence of a promoter to yield a highly viscous, opaque substance resembling grease. He teaches the rearrangement is not possible without the organophosphorus promoter compound. His Example 18 shows that mixed cyclic dimethylsiloxanes, silica filler, and potassium silanolate catalyst do not polymerize without the hexamethylphosphoramide promoter.
Laur in U.S. Pat. No. 3,692,737, issued Sept. 19, 1972, teaches that polydiorganosiloxane gums are usually prepared by alkaline polymerization methods. These methods provide gums which contain alkaline residues which must be carefully neutralized in order to obtain a satisfactory product. Even after careful neutralization, residues in amounts such as 10 to 20 parts per million based on the weight of the gum remain which are then stabilized by the addition of small amounts of finely divided silica. Laur then teaches that fume titanium dioxide used in place of the silica provides a more stable gum.